Is it safe to take erythromycin500mg 3xs a day while breastfeeding a five month old infant

According to www.drugs.com, Erthromycin is considered safe to take while breastfeeding in normal doses.

Just keep an eye on your little one while you are on the medication and watch for increased lethargy, sleeping excessively, not playing as usual etc., and let you doctor know if you notice anything unusual. Make sure only to take the recommended dosing and to use for only the amount of time intended. Hope this helps!

I'm assuming your doctor prescribed this for you and knows you are breastfeeding? Trust your doctor's judgment and go ahead and take your meds. If, however, this is someone else's prescription, beware! Make sure you get the ok from either your doctor or your baby's pediatrician.

As long as your baby is not having any issues from it, most penicillans are safe during pregnancy. The only real issue is if your baby had an allergic reaction to it.
If your doctor prescribed it to you, and is aware you are breastfeeding, trust your doctor ( I don't always say that) but in this case, it is labeled as a safe drug while breastfeeding.